1. Field of the Invention
This invention relates generally to motors and, more particularly, to detection of turn faults in induction motors using estimation of negative sequence impedance trends.
2. Description of the Related Art
A promising technique for induction motor turn fault detection requires measurement of the variation of negative sequence impedance. For example, Joseph Sottile Jr. and Jeffery L. Kohler, in a paper presented at the IEEE/PES Winter 1993 Meeting entitled "An On-Line Method to Detect Incipient Failure of Turn Insulation in Random-Wound Motors," describe a process in which it is feasible to detect turn faults passively on line by trending the negative sequence impedance. Although negative sequence impedance varies with load, the discussion in Kohler et al. assumes that load variation effects are negligible.
In many motors sudden load changes can cause negative impedance variations large enough to interfere with turn fault detections because variations in the range of 0.1% to 1.0% (typically about 0.5%) are sought to indicate fault conditions.
In most motors, although an insulation fault can require a significant period of time to develop, measurable negative sequence impedance effects are not detectable until a turn develops an actual short circuit. Avalanching of a slowly developing insulation weakness can occur in a very short time after a long period of leakage of currents and result in an abrupt change in the negative sequence impedance. Load effects can interfere with turn fault detection measurements and thereby sometimes prevent detection of faults or create false alarms.